The invention relates to a ferro-electric liquid crystal device.
EP-A-0032362 discloses a surface stabilized FLC device which possesses bistability and fast switching. However, in most real devices as cooled from the isotropic phase to the S.sub.c.sup.. phase, instead of a structure consisting of smectic layers normal to the substrate interface as disclosed in EP-A-0032362, a tilted layer structure called a chevron is obtained. Consequently, this gives rise to a relatively small effective bistable switching angle which tends to reduce the overall optical performance of the device. The existence of two different forms of the chevron structure also provides an alignment texture consisting of visible defects called `zig-zags`. These defects usually degrade the appearance of the device.
In order to overcome the above problems of zig-zag defects and a reduced switching angle, a liquid crystal device has been proposed in GB2191870-B in which the surface pre-tilt angle matches the half cone angle of the ferro-electric liquid crystal material. However this technique presently requires the use of a vacuum deposited surface alignment layer in order to generate the required pre-tilt angle of 22.5.degree., which is costly for use in high volume production.
GB2183054 B, discloses the application of a high voltage, low frequency electrical signal to a ferro-electric liquid crystal device to irreversibly reduce the chevron angle so that at the end of the electrical treatment, a layer structure known as the Quasi Bookshelf Geometry (QBG), very similar to the idealised bookshelf, is obtained. Such samples not only have large electrical threshold but also contain defects which scatter light and reduce optical performance.
Based on the relative orientation of the surface pre-tilt, .beta. and the chevron inclination, two forms of tilted layer structure can be realized; Chevron 1 (C1) and Chevron 2 (C2) as disclosed in an article by J. Kanbe et el. Ferro-electrics 14,(1991)P3. The geometrical conditions for these forms are given in FIG. 1 where .delta. and .theta..sub.c are the chevron angle and half of the S.sub.c.sup.. cone angle respectively. The value of .theta..sub.c is usually arranged to be .apprxeq.22.5.degree. and from X-ray diffraction measurements, .delta. is typically 90% of .theta..sub.c i.e. .apprxeq.20.degree..
In devices with low values of the surface pre-tilt angle (.beta.), both C1 and C2 chevrons are stable i.e. both .theta..sub.c +.delta.&gt;.beta. and .theta..sub.c -.delta..ltoreq..beta. can be satisfied respectively. Under these conditions both chevrons are found; zig-zag defects occur at the boundary between the two chevron states and there is very little twist of the director profile in the plane of the substrate through the thickness of the cell. The bistable switching angle (2.theta..sub.s) which is defined as the angle between the projection of the optic axis of tile two memory states onto the substrate plane is given approximately by EQU cos.theta..sub.s .apprxeq.cos.theta..sub.c /cos.delta.
As the chevron angle, .delta., is very similar in value to .theta..sub.c, .theta..sub.s is typically between 6.degree. and 10.degree..
As the surface pre-tilt is increased, zig-zag defects cease to occur because only the C1 chevron is stable i.e. .theta..sub.c +.delta.&gt;.beta. can be satisfied but .theta..sub.c -.delta.&gt;.beta. cannot be satisfied. In addition, the director profile within the bistable domains becomes twisted. When such samples are switched by electrical pulses, two types of bistable states can be obtained: bistable twisted states and bistable planar states. The former correspond to the switching of the director only at the chevron interface. The latter, which correspond to the director switching at the substrate interface, require very large electrical switching pulses (i.e. high voltages and long pulse widths).
In order to achieve large bistable switching angle in low pre-tilt samples, there is disclosed in tile reports (W. J. A. M. Hartman Ferro-electric, 122, 1991 and H. Reiger, et. al. SID 91 Digest (1991), P.137), the application of a low frequency (1-200 Hz) electric field treatment which changes the chevron layer structure. The change is manifested as an increase in the bistable switching angle associated with a decrease in the chevron angle. The resulting structure is known as the "Quasi Bookshelf Geometry" (QBG). The zig-zag defects which are present in the pre-treated (before field treatment) alignment texture vanish as the low frequency square wave signal is applied. As the amplitude of the signal increases, "field line" or "roof-top" defects appear and at a sufficiently high voltage "striped" domains perpendicular to the smectic layer form. After each texture change, the sample requires a progressively larger electrical pulse to switch. In the final "striped" domain texture, latching of the sample would require a pulse three of more times larger than that of the pre-treated texture, but the bistable switching angle increases from a pre-treatment value of say 14.degree. to a value very close to that of the smectic cone angle of 45.degree. after the treatment. The latter leads to improvements in the contrast and transmission of the electro-optic device, but the presence of defects tends to reduce the value of these two parameters.